This disclosure relates to a manufacturing method for preparing polycarbonates, and more particularly, to a manufacturing method for preparing polycarbonates by catalytic melt polycondensation of a dihydroxy compound and a diaryl carbonate in the presence of a catalyst.
Aromatic polycarbonates are used in a variety of applications due to their excellent mechanical and physical properties including, among others, impact and heat resistance, strength and transparency. There are three general processes known for the commercial manufacture of aromatic polycarbonates, which are illustrated in FIG. 1. The conventional interfacial processes, as shown in FIG. 1A, and the phosgene-based melt process, as shown in FIG. 1B, start with the reaction of phosgene with carbon monoxide. The third general process, a xe2x80x9cno phosgenexe2x80x9d melt process as shown in FIG. 1C, was developed to eliminate the use of highly toxic phosgene in the process flow. Of these general methods, the xe2x80x9cno phosgenexe2x80x9d melt process shown is preferred since it prepares polycarbonates less expensively than the interfacial process and avoids the use of highly toxic phosgene. The third process is generally referred to in the art as a melt polycondensation process or transesterification process.
Both types of melt processes shown in FIGS. 1B, and 1C make use of a diarylcarbonate, such as diphenylcarbonate (DPC) as an intermediate, which is polymerized with a dihydric phenol such as bisphenol A (BPA) in the presence of an alkaline catalyst, such as sodium hydroxide, to form a polycarbonate in accordance with the general reaction scheme shown in FIG. 2. This polycarbonate may be extruded or otherwise processed, and may be combined with additives such as dyes and UV stabilizers. Typically, after the initial reaction, the reactor containing the polycarbonate thus obtained is heated and/or reduced in pressure to increase the molecular weight of the polycarbonate and obtain the desired properties in the finished polycarbonate. Once the target properties are obtained, the heating and/or reduction in pressure is discontinued.
In many cases, however, the alkaline catalyst used for the transesterification reaction causes the formation of a cross-linking moiety that results in the production of a branched polycarbonate structure during the heating and/or reduction of pressure. The production of branching species deleteriously influences the mechanical and physical properties of the finished polycarbonate. Thus, it is desirable to repress the formation of the cross-linking moieties caused by the alkaline catalyst to minimize or prevent the formation of branching species in the finished polycarbonate.
A process for the production of a polycarbonate having reduced branching species includes melt polycondensing an aromatic dihydroxy compound and a diarylcarbonate in the presence of an alkaline catalyst in a reactor to produce an intermediate polycarbonate. A branching quencher compound is then added to the intermediate polycarbonate prior to finishing. The molecular weight of the polycarbonate can then increased to obtain a targeted molecular weight by heating and/or reducing the pressure in the reactor, wherein the amount of branching quencher compound added to the polycarbonate is effective to reduce the amount of branching species generated during the heating and/or reduction in pressure.
These and other features will become better understood from the detailed description that is described in conjunction with the accompanying drawings.